1. Field of Art
This invention relates to vertical axis wind turbines.
2. Brief Description of the Prior Art
The Darrieus-type vertical axis wind turbine (VAWT) having its rotating shaft traverse to the air was patented by G. J. M. Darrieus in the United States in 1931 (U.S. Pat. No. 1,835,018). The Darrieus-type vertical axis wind turbine is said to resemble an eggbeater with curved blades connected at both ends to the ends of a rotating shaft. Each blade of the turbine is a symmetric airfoil in cross section and is curved in the shape that a perfectly flexible cable of uniform density and cross section would assume if spun about a vertical axis. The shape of the blade is represented by the Greek word "troposkein" meaning turning rope. With blades having a troposkein confirguration, major stresses are in tension when the rotor is spinning. Thus, rotation of the turbine will not cause the blades to bend nor produce bending stresses in the blades.
The operational principal of the vertical axis wind turbine (VAWT) is analogous to the aerodynamics of a wing (airfoil) as is described in a report entitled The Vertical Axis Wind Turbine "How It Works", SLA-74-0160, December 1974 by B. F. Blackwell of Sandia Laborratories, Albuquerque, N.M. Fluid forces on the airfoil of the VAWT are divided into lift and drag forces. A component of the lift force causes rotor rotation and a component of the drag force opposes rotation. The rotor torque will be positive as long as the driving component of the lift force exceeds the opposing component of the drag force. With such a system the aerodynamic performance is poor at low wind speeds and the blades tend to stall at low rotational speeds. Consequently, some type of auxiliary device such as a motor must usually be employed to start the system.
Interest in Darrieus-type vertical axis wind turbines has been stimulated in recent years by the energy crisis and of the important advantages of such turbines over horizontal machines which include: (1) the turbine accepts wind from all directions and therefore doesn't require costly direction orientating equipment, (2) the turbine doesn't require adjustment of blade pitch to limit maximum output at high wind speeds, (3) the generator, speed reducer and brake do not have to be supported as part of the wind orientating platform, (4) the VAWT blades are supported at both ends and other strategic points as necessary due to excellent aerodynamic performance without pitch change this coupled with the fact that the blades experience significantly less bending stress makes for less expensive and longer lasting blades.
Despite the advancements that have been made in the design of VAWTs and the the inherent advantages of VAWTs over horizontal axis machines, a VAWT is desired which can be even more cost effective in construction, erection, maintenance and operation.
Thereafter, inventors have made several attempts to create a cost effective VAWT that will prosper with the slowly increasing value of energy since the panic of 1973. U.S. Pat. No. 4,449,053 to Kutcher and the Aluminum Company of America (1984) discloses VAWT machines for a wide range of power capacities (22 KW, 60 KW, 112 KW, 300 KW, 500 KW). These VAWTs had an erection hinge for the rotor and upper bearing assemblies and a costly automatic oiling arrangement for the upper bearing. The Kutcher VAWT had a large fixed central support structure that favored the 112 KW and larger machines. The Kutcher VAWTs did not have an erection gin pole therefore requiring the use of a crane for the erection of all machines regardless of power rating. The Kutcher VAWTs did not address guy cable vibration. The Kutcher VAWT was designed with a bias toward large power ratings, massive fixed central support structures, rotor assemblies, rotor bearings, gearboxes, motor-generators and cranes. Nearly all of the Kutcher VAWT components require costly special tooling and enormous first cost financial commitment at what has proven to be little or no production volume to date. The Kutcher U.S. Pat. No. 4,449,053 expired for lack of a maintenance fee.
U.S. Pat. Nos. 4,664,596 and 4,659,284 to Wood address the guy cable vibration and rotor torque resonance by continuously maintaining upper bearing guy cable tension, regardless of temperature, by raising the rotor tube under the pressure of a hydralic lower bearing. The Wood VAWT disclosure seems to suffer the cost peculiarities of large power capacities to include requiring a crane for assembly and erection and special low production components.
U.S. Pat. No. 4,624,624 to Yum has a two blade VAWT with four blade sections. Two blade sections are hinge connected in the center of each blade and at both upper and lower rotor hubs. The upper rotor blade hub can slide up and down a splined rotor tube to form a large number of blade sweep areas and blade maximum diameters. The Yum VAWT claims to be self starting although the explanation is unclear. In the Yum VAWT the blade sweep area is smallest, when the upper blade hub is lowest. This small sweep area is claimed to be a safety position in high winds. It is a valid safety feature. Fortunately fixed sweep area VAWTs are easy to shut down in high winds. This high wind shut down is possible because VAWTs stall so easily at low tip speed ratios. The point is to size the generator and speed reducer so that in winds above 25 miles per hour the VAWT tip speed ratio is always below 4. Cost savings in brake or generator down sizing is more than offset by the moving hub costs. In addition the Yum VAWT moving blade hub must have great support to overcome the great centrifugal forces on the blades in any practical size (above 2000 watts for maximum design wind speed) turbine. This is why most VAWT blade hubs are welded to rotor tubes with relatively large structural moments of inertia.
U.S. Pat. No. 4,364,710 to Campbell discloses a vertical axis windmill with pivoting sail blades. This machine is an excellent example of cable hoisting an entire turbine to a vertical operating position using an erection gin pole and a simple ground mounted winch. Despite this positive feature the Campbell windmill is limited to home use size. This turbine size is due to blade centrifugal forces. Installed high wind survival is also a maintenance problem. Campbell acknowledges these problems in his description (lines 14 through 31).